It is known that laminar flows at a local Reynolds number lower than approximately 200,000 and with a positive pressure gradient (pressure rising in the flow direction) do not engage or stay near a surface but instead separate therefrom.
For this reason, the classic wing profile with a smooth or rough surface is consequently only usable above a minimum Reynolds number. For a Reynolds number below this minimum, the laminar boundary layer is separated from the solid surface on the suction side of the wing profile in the vicinity of the profile leading edge. This fact not only limits the use of airfoils, propellers, etc., but also the stirring action of stirrers, with which, in relatively viscous media, laminar flows are produced. This fact also limits the pressure recovery in diffusers.